1. Field of the Invention
The present invention relates to a field emission type cold electron emission device that emits electrons under a large external electric field. Such a cold electron emission device is widely applicable to ultrafast, highly environmental resistant electronic devices, various sensors, image display units like flat-panel displays, electron microscopes and various instruments using an electron beam.
2. Description of Related Art
FIG. 4 shows a typical cold electron emission device with an emitter consisting of a semiconductor (K. Betsui, Technical Digest 4th Int. Vacuum Microelectronics Conference, Nagahama, 1991, p. 26). The device comprises an emitter 2 with a sharp tip formed on an n- or p-type single crystal silicon substrate 1, an insulating layer 3 and a lead electrode 4 formed on the insulating layer 3 in such a manner that it surrounds the tip of the emitter 2. The emitter 2 is formed by a sharpening technique using plasma etching in conjunction with thermal oxidation. This type is the mainstream of the cold electron emission device at present because it has highly reproducible structure and provides a large emission current at a rather low voltage owing to the sharp tip of the emitter 2.
The field emission type cold electron emission device, however, has a problem of large fluctuations in its emission current which sharply reduces and increases according to the lapse of time, thereby breaking the device in the worst case. This also applies to the device as shown in FIG. 4, which hinders the practical use of the device. This phenomenon is mainly due to the fact that the work function of the emitter tip greatly fluctuates in space and time because of the adsorption of residual gas in the ambient and contamination in the fabrication process. The foregoing conventional example, however, does not devise any countermeasures against such fluctuations in the current.
In order to solve such a problem of the cold electron emission device, two strategies can be employed: one of them is to stabilize the work function of the emitter tip; and the other is to control the emission current. With regard to the emission current control, remarkable techniques have been proposed recently (A. Ting, et al., Technical Digest 4th Int. Vacuum Microelectronics Conference, Nagahama, 1991, p. 200; and K. Yokoo, et al., Technical Digest 7th Int. Vacuum Microelectronics Conference, Grenoble, France, 1994, p. 58).
FIG. 5 shows a device of this type. In FIG. 5, the magnitude of the current emitted from the tip of an emitter 12 is controlled by a field effect transistor (FET) connected in series with the emitter 12 having the same structure as the emitter 2 in FIG. 4. As shown in FIG. 5, the emitter 12 is provided on a p-type silicon substrate 11. The emitter 12 is an n-type, and its extended portion forms an n-type drain 13 of the FET. An n-type source 14 is arranged on the p-type substrate 11, and a source electrode 15 is provided on the source 14. Part of an insulating layer 17 between the substrate 11 and a lead electrode 16 is thinned to form a gate insulating layer 18, on which a gate electrode 19 is arranged. In this device, the gate voltage of the FET uniquely determines the drain current of the FET, which is equal to the emission current from the emitter tip. This means that the gate voltage also uniquely controls the emission current.
The FET controlled type cold electron emission device can solve in principle the problem involved in the conventional cold electron emission device because it can precisely control its current.
The conventional technique, however, must have the FET for controlling the current in addition to the emitter 12. In this case, the FET will generally occupy a larger area than the emitter 12 on a horizontal plane, even though it is formed around the emitter tip, that is, around the base of the protrusion. This will greatly increase the area per element, resulting in a remarkable reduction in the integration density of the emitters. The MOSFET in the element as shown in FIG. 5 must have a very long narrow gate because the current emitted from the single emitter 12 is very small on the order of less than one microampere. For example, to implement the drain current Id less than one microampere, it is necessary for the gate whose width is W and length L to satisfy the relation of L&gt;100W considering that the drain current Id is proportional to W/L. This means that when W is 1-2 microns, L must be 100-200 microns, and that an area of several tens of square microns is required for each emitter. Furthermore, additional wiring to the source and gate electrode of the FET will further reduce the integration density of the emitters. Moreover, the FET, which must be formed separately from the emitter substantially complicates the fabrication process, thereby reducing manufacturing yield.
In order to solve the problems of the foregoing two conventional techniques, the inventors of the present application propose a device disclosed in Japanese Patent Application Laid-open No. 9-63466 (1997) laid-opened on Mar. 7, 1997. FIG. 6 shows the structure of the device. The device has its emitter protrusion 24 provided on one of two n-type regions 22 and 23 which are arranged on a p-substrate 21, a source electrode 25 arranged on the other of them, and an electrode 26 with an opening surrounding the emitter protrusion 24 arranged on an insulating layer 27 which is arranged across the two n-type regions 22 and 23. The device, when considering the emitter as a drain, has a well-known MOSFET structure, and hence the electrode 26 has the function of the gate electrode for controlling the channel current of the MOSFET, in addition to the function of the lead electrode that induces the field emission by generating a large electric field at the tip of the emitter protrusion 24. In this case, the field emission current increases on an exponential curve of the voltage applied to the electrode 26, whereas the channel current increases on a square curve thereof. Accordingly, supplying the electrode 26 with a rather high voltage enables the field emission current to be controlled to become greater than the channel current. Thus, the field emission current is limited by the channel current, which makes it possible to emit a constant current from the emitter tip as in the conventional example as shown in FIG. 5.
As described above, it is difficult for the conventional technique as shown in FIG. 4 to be put into practical use because its emission current is very unstable. Although the conventional technique as shown in FIG. 5 can achieve a stable current, its integration density of the emitters substantially reduces, and the number of its fabrication steps increases owing to the complicated fabrication process, thereby reducing its yield and increasing its cost. Although the conventional technique as shown in FIG. 6 is sophisticated in that its structure is simple and has a current stabilizing function, it has a drawback that it must have n-type impurities introduced into both the source and drain (emitter itself) regions. Generally speaking, it is not easy to introduce impurities evenly into a structure like the emitter with a sharp tip, which results in an increase in the cost. This causes a serious problem in application devices such as a flat-panel display.